Could Install OpenWrt on a Lenovo M600 mini PC?

I am considering flashing openwrt into this windows based mini pc adding a wifi adapter and switch to turn it into access point but not sure if its compatible please help??

It will work with x86/x64 image. Check documention for those.

What @KOA said is correct -- an x86/64 image would be the way to go regarding a compatible build of OpenWrt.

However, keep in mind that using this device as an AP will not be a particularly good option. The on-board wifi (if any) is probably not great, and many USB wifi adapters do not support AP mode. Those that do are functional, but usually low performance 1x1 radio systems which are not designed for efficient operation/high performance with multiple client connections, and they also typically have small antennas* limiting range (*even those that have physically large external antennas may actually have a very small physical/functional antenna element surrounded by a large plastic shell).

Also consider that these systems may draw north of 50W, which is ~3-4x the power required by a high quality purpose build/dedicated AP.

x86 systems are often excellent routers but the wifi performance as an AP will be poor.